In recent years, semiconductor chips are generating heat in increased amounts accompanying the trend toward higher degree of integration and higher operation speed of semiconductor chips, causing an increase in the occurrence of defects stemming from the discrepancy of thermal expansion between the semiconductor chip and the molding compound for sealing it or a circuit board (particularly, laminated circuit board) mounting the semiconductor chip thereon. This tendency is requiring an increase in the amount of the filler added to the molding compound for semiconductor chips or added to the resin used for forming the insulating layer of the laminated board. Addition of the filler in an increased amount enables the generated heat to be efficiently radiated and, further, lowers the coefficient of thermal expansion so as to approach the coefficient of thermal expansion of the semiconductor chips. For the resin compounds used for sealing liquid crystal displays, too, it has been desired to add the filler in an increased amount in order to improve the reliability for sealing the liquid crystal displays.
As a filler used for a molding compound for semiconductor devices, there have been known fine dry silica particles such as fumed silica (usually called dry silica) prepared by the flame hydrolysis of chlorosilane (see patent document 1).
The fumed silica works to impart a high degree of viscosity even if it is added in small amounts to the resin in a liquid state (molten-state or solution-state). If added in an increased amount, therefore, it becomes difficult to form the resin (resin composition). Namely, limitation is imposed on the amount of using the fumed silica.
Further, the fumed silica contains, as an impurity, chlorine that stems from the chlorosilane which is the starting material and brings about a defect of corroding metal wiring and the like.
In order to make it possible to add the fumed silica in large amounts avoiding its effect of highly imparting the viscosity, it can be contrived to use spherical silica having an average particle size of about 1 μm as a filler. In this case, the filling rate of the filler can be surely increased while suppressing a rise in the viscosity of the resin. However, the spherical silica not only much contains coarse particles of several microns or larger stemming from the production method but also exhibits strong aggregating property and much contains aggregated particles which cannot be easily dispersed in the resin. As a result, the resin compound to which the spherical silica is much added exhibits poor gap permeability, clogs among the wirings, and deteriorates the reliability of equipment to which the resin compound is applied.
Further, the resin compound for semiconductor devices must have decreased local dispersion in the coefficient of thermal expansion in order to improve reliability of the electronic equipment which uses the semiconductor sealing material. However, the filler material having a broad particle size distribution disperses unhomogeneously in the resin composition, developing local dispersion in the coefficient of thermal expansion and forming portions where the coefficient of thermal expansion greatly differs.
Further, the above-mentioned fumed silica is used as an external additive for toner used in the electrophotography such as a copier or a laser printer in order to impart fluidity or to control the amount of electric charge of the toner (see patent document 2).
Toner resin particles of a small diameter have been used and, besides, a toner resin having a low softening temperature has been used in the modern electrophotography featuring finer pictures, higher picture quality and high speed. Therefore, the toner particles tend to melt-adhere together lowering the fluidity thereof. Accordingly, the toner additive for covering the surfaces of the toner particles must have the effect for imparting fluidity as well as the anti-blocking effect more than ever before.
However, the fine fumed silica particles used as the toner additive have a branched structure in which the primary particles are melt-adhered, exhibit a less effect for imparting fluidity than the individual spherical particles and, further, have a small primary particle size allowing silica particles to sink below the surfaces of the toner resin particles due to external stress such as stirring, without playing the role of antiblocking for extended periods of time, allowing the fluidity of the toner to gradually decrease, making it difficult to scratch off the residual toner by the cleaning blade and causing inconveniences such as a decreased transfer efficiency at the time of forming the picture and a decreased picture equality due to filming phenomenon. In this case, the above-mentioned inconveniences can be avoided by using an external additive having a large particle size. However, a too large particle size decreases the effect for improving the fluidity. Therefore, sinking in the toner resin particles is not prevented even if there is used a toner additive having too large particle sizes. Further, the toner additive containing hard and coarse particles of not smaller than several microns becomes a cause of impairing the durability of the photosensitive material drum due to abrasion.
Further, the toner additive must have a function for controlling the electric charge of the toner particles. However, the toner additive having a broad particle size distribution disperses or adheres unhomogeneously arousing a problem in controlling the electric charge and causing a decrease in the picture quality. Even when metals such as iron and sodium, as well as chlorine are contained as impurities, the amount of electric charge decreases and it becomes difficult to control the electric charge.
In order to overcome the problems in the above resin filler and in the toner additive, the fine silica particles used for the above applications must have the following properties.    (a) Do not contain coarse particles of several microns or larger.    (b) Have a sharp particle size distribution.    (c) Contain little impurities such as chlorine.
Generally, the following five methods have been known for producing fine silica particles.    (1) Sol-gel method (see patent document 3).    (2) Flame hydrolysis of chlorosilane (see patent document 4).    (3) Combustion of silicon powder (see patent document 5).    (4) Spray combustion of a liquid siloxane without containing halogen in the molecules thereof (see patent document 6).    (5) Diffusion combustion of siloxane gas without containing halogen in the molecules thereof (see patent document 7).
In the case of the sol-gel method of (1) above, so-called mono-dispersed particles are obtained enabling the particle size and the particle size distribution thereof to be easily controlled. However, the particles aggregate firmly in the steps of drying and firing for removing water contained in the silica arousing a problem of formation of coarse particles.
In the case of the flame hydrolysis of chlorosilane of (2) above, a chlorine compound is by-produced and is adsorbed by silica that is formed accounting for the residence of chlorine in an amount of about several tens of ppm. Besides, the flame temperature is low due to the use of chlorosilane as the starting material and the region becomes small where the formed particles collide and grow together. As a result, fine silica particles do not collide together to grow and it becomes difficult to obtain silica particles of a size adapted to the use as a filler. Even if particles were obtained having desired sizes, there are much contained poorly dispersing particles stemming from the melt-adhesion of primary particles.
In the case of the combustion of the silicon powder of (3), an advantage is that no halogen compound is contained in the starting material. To control the average particle size of the obtained silica, however, it becomes necessary to control the silicon concentration of the starting material in the flame, and feeding the starting material maintaining stability is a prerequisite. However, since the starting material (silicon powder) is a solid powder, it is difficult to feed the starting material at a constant rate maintaining stability though the feeding rate can be increased. Due to fluctuation in the feed of the starting material, therefore, the concentration of silica source varies in the flame and it becomes difficult to obtain fine silica particles having a sharp particle size distribution without containing coarse particles. Further, even if the starting material can be fed maintaining stability, the particle size distribution stemming from the starting powder tends to vary or the concentration tends to vary due to the deviation of the starting material in the flame making it difficult to obtain fine silica particles having a sharp particle size distribution without containing coarse particles.
In the case of the spray combustion of (4), the starting material that is used is a liquid (liquid siloxane) and can be easily fed maintaining stability accompanied, however, by the difficulty in adjusting the droplets that are sprayed to possess a predetermined size. Besides, the liquid droplets may be present in some places but may not be present in other places causing a difference in the concentration of silica source in the flame and making it difficult to obtain fine silica particles having a sharp particle size distribution without containing coarse particles.
Therefore, the methods (1) to (4) are not adapted to obtaining silica particles having the above-mentioned properties (a) to (c). According to the diffused combustion of (5), the siloxane without containing halogen in the molecules thereof is gasified and is quantitatively fed into a burner to form silica in the flame. The method (5), usually, employs means for burning the siloxane by introducing a siloxane gas into the burner together with a carrier gas such as nitrogen, and diffuse-mixing a combustion-sustaining gas (oxygen, air, external air, etc.) separately introduced into the burner therewith at the outlet of the burner.
According to the above diffusion combustion method, the mixed state of the siloxane gas and the combustion-sustaining gas greatly affects the state of combustion of the siloxane or the formation and growth of the fine silica particles making it, however, very difficult to control the mixing of the gases in a diffused state and, therefore, difficult to control the particle size distribution of the obtained fine silica particles. Further, without separately introducing oxygen in an excess amount into the burner, the siloxane does not burn completely, carbon soot forms, and fine silica particles are not obtained.
As a method of controlling the state of mixing the siloxane gas and the combustion-sustaining gas yet solving the problem of incomplete combustion, the present inventors have proposed a method of burning the siloxane gas by mixing the siloxane and the combustion-sustaining gas together in advance and by introducing the mixed gas into the burner as described in a patent document 7. This method surely prevents the incomplete combustion of the siloxane. However, no method has yet been known to control properties such as particle size distribution of the obtained fine silica particles, and the silica particles having the above properties (a) to (c) have not yet been obtained.
When the fine silica particles include coarse particles, it can be contrived to remove the coarse particles by the classifying operation. However, no technology is yet available for removing particles of about 3 to 5 μm by dry classification method such as cyclone or pneumatic classification. Therefore, the fine silica particles must be classified by the wet method such as wet sieving or classification by hydraulic elutriation. These means, however, require the step of drying fine particles causing the particles to be strongly aggregated together at the time of drying and rather forming coarse particles. After all, there is at present no method of controlling the fine silica particles for their particle size distribution relying upon the classification.
Patent document 1: JP-A-1-161065
Patent document 2: JP-A-2002-116575
Patent document 3: JP-A-4-21515
Patent document 4: JP-A-2002-3213
Patent document 5: JP-A-60-255602
Patent document 6: JP-A-2002-60214
Patent document 7: JP-A-2002-114510